Does Your Ruler Measure Up?

Small plastic rulers were thought to be imprecise by metrologists; now, researchers have shown that most meet the official standards, but that these are ‘shockingly poor’ themselves

Complimentary small plastic rulers, being imprecise, innacurate, flimsy and defaced with advertising, draw only a measured amount of respect from metrologists. In 1994, two metrologists took measures to see just how much respect the rulers deserve.

Metrologists are the people who come up with more accurate, more precise ways to measure things.

The metrology community incessantly tussles about new standard definitions for the intimidatingly important, never-quite-as-good-as-they-ideally-could-be standards – most famously, the kilogram, the metre and the second.

The father-and-son team of TD Doiron and DT Doiron looked, briefly, at a neglected standard. Their report, called Length Metrology of Complimentary Small Plastic Rulers, drew some measure of interest at the Measurement Science Conference in Pasadena, California.

Theodore Doiron was a member of the Dimensional Metrology Group at the National Institute of Standards and Technology. Daniel was, at the time, a high school student.

The Doiron/Doiron report implies two simultaneous and opposite truths. Metrologists sometimes express contempt for small plastic rulers (known in the trade as SPRs), because they are made of cheap polystyrene and manufactured to loose tolerances. But metrologists also, down deep, harbour respect for these stylish, useful, slim, flat-bottomed objects, with the four straight-edge working surfaces and the top that boasts a sufficiency of both inked markings and raised graduations, said graduations being located at the outer edges of the beveled top sides.

The Doirons explain this ambivalent attitude:

“There are virtually no active scientists or engineers who do not have a number of SPRs in their desks which are used continually for developing the earliest and most basic designs of virtually every object manufactured. A quick survey of engineers will show that these early sketches, the very basis of our manufacturing economy, are largely dependent on the use of SPRs. While there is a federal standard for plastic rulers, Federal Specification GG-R-001200-1967 and the newer A-A-563 (1981), there has never been a systematic study of the metrology of this basic tool of the national measurement system.”

Doiron and Doiron studied 50 rulers they had “collected over a long period of time at conferences and from colleagues”. They discovered that the government specification was itself so shockingly poor that they could point to a key passage and say: “We cannot figure out what this statement means.”

After measuring things as best they could (and being good metrologists, they could measure things well indeed) the Doirons reached a pair of conclusions. First, that most of the complimentary small plastic rulers “quite easily” met the official (albeit murky) standard. Second, that “the older the ruler” was, the more accurate it was likely to be.

The National Institute of Standards and Technology (Nist) itself, an official told me, once ordered a batch of complimentary small plastic rulers that turned out, upon arrival, to be wretchedly calibrated. As a measure of caution (they have a reputation to protect), and perhaps with some umbrage and embarrassment, Nist returned them to the manufacturer.
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